If we ever achieve manned missions to the stars, one of the assumptions is that we will find planets much like Earth that we might live on and colonize. But what if the assumption is flawed? There are surely many Earth analogues in the Milky Way, but we don’t know how widely they are spaced, and a near-miss isn’t necessarily helpful, as both Mars and Venus attest. People like Robert Zubrin continue to advocate terraforming as a solution for Mars, and it may well happen one day, but supposing we get to another star, would we have the moral right to terraform a world with living creatures on it, even if they didn’t meet our criteria for intelligence?

Robert Kennedy (The Ultimax Group), working with colleagues Kenneth Roy and David Fields, has been pondering these issues and went through a possible solution at the recent Tennessee Valley Interstellar Workshop in Oak Ridge. If we stop worrying about Earth analogues, a range of interesting possibilities open up, as our own Solar System illustrates. We have small planets like Mars, along with what may be a huge number of dwarf planets. We also have moons in a wide range of sizes around the gas giants. Suppose we could transform such worlds by building a spherical shell of matter around them, totally enclosing an atmosphere and living ecosystem?

Beyond the Habitable Zone

The idea seems outrageous, but Centauri Dreams readers are familiar with even more gigantic concepts like Dyson shells, engineering on levels that would require a Solar System-wide infrastructure and a Kardashev Type II-level civilization to build. If we extrapolate advancing technologies that can do gigantic things, we can consider creating an Earth-like environment (in most ways) under a shell that protects the inhabitants from radiation and provides a self-enclosed ecology. The question of a ‘habitable zone’ would disappear because artificial lighting and temperature control would be built in, and the wild card would be gravity, which would depend on which bodies were selected for enclosure. Most would offer gravity only a fraction that of Earth’s.

Kennedy and team wrote a paper for JBIS in 2009 that lays all this out. Working the math on spherical shells, they ponder the fact that if the objective is to contain a 14.7 psi Earth-normal atmosphere, such a shell would experience the same kind of pressure-induced tension found in a balloon. Assume one atmosphere of pressure at the underside of the shell and vacuum above it, and it is possible to choose a shell thickness so that the compressive stress of gravity cancels out the atmosphere-induced tensile stress in the shell. A shell made completely of steel, for example, built to enclose a world 20 kilometers above its surface, would need to be 1.31 meters thick if enclosing the Earth, and 8.05 meters thick if enclosing the Moon.

Moreover, the shell mass used is there simply to create compressive force — opposing the pressure of the atmosphere within the shell — and can be no more than dead weight. The authors figure that enclosing the Earth’s Moon could be done with no more than a 1-meter thick layer of steel if it incorporated 62 meters of regolith on top of it, with open-ended combinations of steel, ice, dirt and rock possible for the job:

It is not actually necessary to use a metal such as iron or steel. Stony materials such as concrete can handle a lot of compression. A strong fabric material that is airtight and in slight tension could be used to support the mass of the shell, which could be mainly rocks and dirt.

The authors contend that a shell with mass equally distributed across the surface of the shell will be stable with respect to the more massive body at the center of the shell:

If the central mass is displaced a given distance inside the shell, gravity will act to restore the shell’s original position with respect to that body. Such is not true for a ring. If there were no way to damp the movement, the shell would oscillate back and forth. A viscous atmosphere will tend to dampen oscillations until the mass center is once again congruent with the center of the shell.

The Riches of Ceres

Now consider the asteroid Ceres. Here the shell, depending on which mass estimate for Ceres we choose, would have to range (if made of steel) from 45.2 to 90.4 meters in thickness — this is the amount of mass that would be necessary to hold an Earth-normal atmosphere. This is one thick covering, providing enough shielding to survive a nearby supernova. Assume you have a terraformed Ceres that is half ocean and you wind up with enough dry land area to approximate the area of Indonesia, on a world where gravity is 1.5 percent that of Earth. Could a human colony survive in conditions of micro-gravity? At this point we simply don’t know the answer.

But think about the scenario for a moment. In an enclosed Ceres, climate is a design variable and lighting can be adjusted to approximate whatever day/night cycle the occupants desire. Imagine the underside of the shell as the urban area, a place where residents live in housing that overlooks the spectacular vista of the interior, which could be maintained as farmland or a nature preserve filled with whatever species the designers choose to introduce. With normal atmospheric pressures and light gravity, human-powered flight would always be an option. The outside of the shell would be devoted to heavy industry for manufacturing and power plants.

Taken to an extreme, we get this:

… the subterranean zones of small celestial bodies would offer vast – virtually unlimited – cubic for support functions and resource extraction. Consider that the interior of Ceres – half a billion cubic kilometers – could contain almost exactly the same working volume as a world-spanning city which packed the entire surface of Earth, oceans included, with billions of 1 km high skyscrapers, each the rival of Burj Dubai. In the light gravity of Ceres, every bit of that volume would be easily reachable and cheaply exploitable, unlike the deep wells and mines of Earth. A shell world might well be the richest planet in its solar system, once the huge cost of englobement was paid off.

Building for Safety (and Aesthetics)

Numerous dangers could beset a shell world, including many that already threaten our planet, such as the impact of large asteroids, but we could avoid some problems — volcanoes and earthquakes spring to mind — if we choose or build worlds without plate tectonics, and issues like solar flares would have little effect given the shielding the shell world’s inhabitants could rely on. A rupture in the shell would be a hazard, but a small shell world like Ceres would have a shallower gravity well than Earth and be less likely to draw in an asteroid. Moreover, any shell world would include the kind of planetary defense systems that a civilization capable of building the shell in the first place would be able to deploy. Shell maintenance, safety and improvement would doubtless be an ongoing project.

The paper works through one possible construction scenario involving the Moon and considers the massive amounts of energy required to move the needed terraforming materials (roughly one quadrillion tons), obviously requiring huge advances in energy production and space transportation. But it’s a fascinating vista, one that sees the creation of hanging cities on the underside of a shell that represents an area equal to four times the area of the United States. The surface of the re-made Moon can be tuned up to be as Earth-like as we choose to make it, the entire project taking hundreds and more likely thousands of years to see to completion.

The presence of hanging cities will diminish the required surface loading by inert material. Lighting would be artificial, with solar energy (assuming the shell world is near a star) powering up the lights, or power plants on the surface of the shell doing the job if the world were built in deep space. The paper argues that shell worlds all the way out to the Kuiper Belt could have an Earth-like insolation, ecology and diurnal cycle. And imagine this:

Existing electro-luminescent displays (ELD) only provide about 1-2 W/m2 of radiant energy in various colors from red to blue-green, but their state of the art (brightness, efficiency, cost) is rapidly advancing… Since ELD materials are presently available in all three primary colors and can be subdivided into addressable segments, we can imagine a pixelated ceiling of video wallpaper simulating the natural sky of Earth (clouds, sunsets, stars, etc.) or generating any arbitrary scene. The postindustrial motto “everything is media” means art can reach its fullest expression in the canvas of a shell world.

If we do become the Type II civilization capable of building such structures, we’ve not only opened up numerous worlds within our own system for colonization, but have also gained the experience needed for constructing stable generation ships for long-duration interstellar flight. And because shell worlds could be located anywhere a suitable moon or planet is found, we should consider the possibility that alien civilizations may already have constructed such worlds around red dwarf stars or even brown dwarfs, which may outnumber all other kinds of stars. The traditional concept of a habitable zone may not be the marker we’ve always assumed it to be, with prospects for SETI extending to worlds that would not before now have gained our attention.

The paper is Roy, Kennedy and Fields, “Shell Worlds: An Approach to Terraforming Moons, Small Planets and Plutoids,” JBIS Vol. 62 (2009), pp. 32-38. If you’re a science fiction writer in search of a setting, you must read this paper. I’ll also let everyone know when the Oak Ridge presentations become available online so you can see Robert Kennedy’s talk and slides.